Non-volatile memory can be implemented using a storage transistor having a floating gate. The floating gate is located between a control gate and a substrate, and unlike the control gate, the floating gate is typically not connected to a word or bit line. The logic state of the storage transistor can be altered by injecting electrons into or removing electrons from the floating gate to cause a substantial shift in the threshold voltage of the storage transistor and thus, turn the storage transistor ON or OFF. For positive logic, the state of the storage transistor is referred to as “1” or “0” state when it is ON or OFF, respectively. On the other hand, the state of the storage transistor is referred to as “1” or “0” state when it is OFF or ON, respectively, for negative logic. Typically, conventional electrically erasable programmable read-only memories (EEPROMs) are negative logic, and erasable programmable read only memories (EPROMs) or Flash Memories are positive logic.
Under high gate voltages, electrons can tunnel through tunnel dielectric layers, e.g., silicon dioxide, to move into or out of the floating gate. Hot electron injection requires both high gate and high drain voltage to gain sufficient energy to jump the silicon-silicon dioxide energy barrier, penetrating the oxide and flowing to the floating gate, which is surrounded by an oxide layer.